Transistor inverter circuit for supplying constant current output

ABSTRACT

A constant current power supply for providing constant current to loads having low power requirements and variable resistance characteristic in a reliable, less complex operating mode. A tuned collector, resonant feedback, saturated current oscillator circuit is enabled to provide an alternating current output which is proportional to an adjustable DC control voltage, and, by proper selection of the feedback amplitude in the oscillator circuit, the output current to a resistance load can be made independent of the load resistance.

United States Patent 1 91 A Wallace I -[22] Filed:

[ TRANSISTOR INVERTER CIRCUIT FOR SUPPLYING CONSTANT CURRENT OUTPUT '[75] Inventor: Kenneth A. Wallace, Columbus.

I Ohio [73] Assignee: North Electric Company, Galion.

- Ohio Mar. 16,1972

21 Appl. No.1 235,335

52 us. 131 331/109, 321/2, 321/17, 321/18, 331/113 A, 331/117 R, 331/183 51 1m. (:1 H02m 1/08, l-l02m 3/32, 1103b 3/02 [58] Field of Search....- 331/113 A, 117 R,

[56] References Cited I UNITED'STATES PATENTS 7 2,919,412 12/1959 Tyler'..; 331/117 X 3,305,760 2/1967 Davisetal. 321/18 )1 DC SOURCE IO 1 Nov. 27,1973

6/1967 Gardner et al 321/18 X 11/1966 Marlow 331/109 Primary Examiner-Roy Lake Assistant Examiner-Siegfried H. Grimm Attorney-Johnson, Dienner, Enirich, Verbeck '&

Wagner [57] ABSTRACT i A constant current power supply for providing constant current to loads having low power requirements and variable resistance characteristic in a reliable, less complex operating mode. A tuned collector, resonant feedback, saturated current oscillator circuit is enabled to provide an alternating current output which is proportional to an adjustable DC control voltage, and, by proper selection of the feedback amplitude'in the oscillator circuit, the output current to a resistance load can be made independent of the load resistance.

a Claims, 5 Drawing Figures CONTROL CKT.

CONSTANT CURRENT OSCILLATOR LOAD 1B age.

TRANSISTOR INVERTER CIRCUIT FOR SUPPLYING CONSTANT CURRENT OUTPUT BACKGROUND OF THE INVENTION 1. Field of the Invention; The invention relates to a self-oscillating AC constant current source for low power loads having a variable resistance characteristic. 2. Description of the Prior Art In certain field applications, there is a need for an adjustable, AC constantcurrent power supply for loadswhich have a variable resistance characteristic andrelatively low power requirements at high voltages. Electrostatic copying mabe minimized by providing a constant current input to the corona emitting device, whereby variations in the ambient conditions'which result in the variable resistance values of the corona emitting device will not significantly affect the current supply to the corona emitting device. The provision of an inexpensive, reliable constant currentcircuit for such equipment, however, has inherent problems} That is, in addition to the changing load current which occurs with a change in the resistance of the corona emitting device as ambient conditions vary, there is a further. flow which occurs over the equivalent line capacity. It is, of course, necessary in the provision of a constant source to ascertain the current requirements for such conditions; However, since the line capacity current and the load current return through a common ground, the currentdemands of the circuit cannot be readily sensed. r r

The problem is further aggravated by the fact that the corona emitting device is a high voltage device and the value of the line capacity is therefore of a significant value. In addition, since the value of the capacity is a function of wiring devices and the like, the tolerance is quite high. This rather large value of capacity tolerance, in turn, makes it very difficult to predict the line capacity current from measurements of the output volt- The problem of the changing resistance value of the corona emitting device with ambient conditions and the difficulties of ascertaining current flow required to accommodate the line capacity of the circuittalong with the need to provide a basic circuit which will be adjustable for use with the different line capacities of the copiers as manufactured), has madedifficult the provision of a reliable, inexpensive circuit which is capable of maintaining a constant current to the corona emitting device of an electrostatic copier.-

SUMMARY OF THE INVENTION It is an object of the present invention, therefore, to provide a power supply source for use in constant current applications requiring a relatively low power output with reasonable drift and regulation requirements chines. which use corona emitting devices as the radiatwhich is reliable, inexpensive, and utilizes a minimum of components. It is one specific object of the invention to provide such type circuit for use in the supply of consta'nt current at a relatively high voltage.

According to the invention, such arrangement comprises a transformer having a center-tapped primary winding, a center-tapped feedback winding and a secondarywinding. first and second transistor have collector elements connected to opposite ends of the two sections of the primary winding in an oscillatory mode, the frequency of oscillation being determined by a tank circuit comprised of the magnetizing'inductance L, of the transformer, and a fixed capacitance C0v connected across the transformer primary or secondary winding. In high voltage applications the distributed capacity C in i the transformer windings and output wiring must also be considered along with the fixed capacitance C.,, thusly: v v

back winding, the output of which is fed to the base elements of the transistor pair. The output current of the transistors is, thus basically determined by the value of the control voltage, the baseto emitter drop of the transistor, the feedback voltage and the resistance'R in the emitter circuit of the transistors. Adjustment of the value of the control voltage E effects corresponding adjustment of the amplitude of the waveform output by the oscillator circuit, and thereby the value of l the constant current supplied to the load.

According to the invention, the transistor "collector output is comprised of a waveform having two sinusoidal components. The first component which is determined by the control voltage is of a constant value which provides the normal load current requirements. The second sinusoidal component of current is proportional to the output which appears across the secondary winding of the transformer, the amplitude of which component has been selected to compensate for the core loss resistance of the transformer. Variation in the voltage across the secondary winding of the transformer is sensed by the center-tapped feedback winding which responsively adjusts the value of the feedback voltage e, to the base elements of the transistor to therebyadjust the value of the current output of the transistor pair, whereby the output current which flows over the load is maintained at a constant value.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 sets forth one embodiment of a constant current supply source of the invention;

FIG. 2A sets forth selected waveforms which occur in the circuit of FIG. 1;

FIG. 28 sets forth, the equivalent circuit of the supply source shown in FIG. 1;

FIG. 3 is a modification of the basic circuit of FIG. 1 which provides a regulated DC output; and

FIG. 4 is a modification of the basic circuit of FIG. 1 which includes a feedback circuit which-derives a feedback control signal from the output circuit.

CIRCUIT DESCRIPTION bodiment comprised a corona-emitting device of the type commercially used in electrostatic copiers manufactured by Xerox Company, Rochester, New York,

which has a resistance value R, which varies with ambient conditions. The output of saturated current oscillator 16 to such type load in one embodiment was in the order of 100-400 microamps at a 4 KV 6.8 KV at 600 Hertz. I

As shown in FIG. 1, saturable current oscillator 16 comprises a high voltage transformer 26 having a primary winding 20 which in one embodiment comprised 143 turns of No. '29 wire, a high voltage secondary winding 22 comprised of 35,385 turns of No. 39 wire, and a centertapped feedback winding 21 comprised of five turns of No. 29 wire. Primary winding 20 has a center tap which is connected over input conductor 12 to the positive terminal of DC source 10. The terminal ends of primary winding 20 are connected, respec- "tively, over conductors 28A, 19 to the collector elements of first and second power drive transistors 28, 30. The emitters of transistors 28, 30 are in turn connected over a common resistor 31 (r to battery ground. Resistor 31 in the disclosed embodiment is in the order of ohms. Transistors 28, 30 may comprise 2N3767 NPN transistors which are commercially available from Motorola Corporation, Phoenix, Ariz.

Filter capacitors 32, 33 may be connected across the upper and lower sections of winding to eliminate possible spurious oscillation of the transistors 28, 30.

Feedback winding 21 on high voltage transofrmer 26 has its terminal ends connected overconductors 27, 29 to the base elements of transistors 28, 30 respectively. The centertap on feedback winding 21. is connected to the output of a control circuit 34 which supplies a constant voltage E over conductor l3to the center tap of the feedback winding 21. Control circuit 34 which may be one of a'number of conventional adjustable constant voltage sources is shown in one embodiment as comprising a series-connected resistors 35 and Zener diode 36 which may be of the type available as a 1N4372A connected across the 45 volt output of DC source 10, and series-connected resistor 37 and diode 38 connected across diode 36. The movable arm on adjustable resistor 37 is connected over conductor 13 to the junction of the center tap of the feedback winding 21 and also over capacitor 39 to ground. Capacitor 39 as thus connected operates to filter unwanted frequencies from the feedback winding.

Secondary winding 22 on high voltage transformer 26 is connected to load 18. Load 18, as noted heretofore, may comprise a corona emitting device.

Capacitor 23 when connected across secondary winding 20 is in the order of 150 When capacitor 23 is connected across the primary winding 20 as indicated by the dotted lines, the value of capacitor 23 is in the order of luf. The two vertical lines shown between the primary and secondary windings 20, 22 schematically represent the core of transformer 26.

The phase relationship between the primary winding 20 and feedback winding 21' are indicated by the dots in FIG. 1. The feedback winding 21 provides positive feedback voltage over conductors 27, 29 to transistors 28 and 30 and causes the oscillator circuit 16 to oscillate at a frequency given by where C is the value of capacitor 23 when connected across secondarywinding 22 and L isthe magnetiZing inductance of transformer 26 referred to the secondary winding 22. In high voltage applications C also includes the capacitance value of thesecondary winding-22 of transformer 26 and the wire capacitance between conductors 24, 25. The magnetizing inductance can be varied byadjusting the gap in the transformer core. The value of capacitor 23 and/or the magnetic inductance L may be varied to obtain the desired operating frequency.

Once the oscillator circuit16 is oscillating, the feedback voltage provided by winding 21 will cause transistors 28'and 30 to conduct current on alternate half cycles. The current flow at such time will occur from the positive terminal of DC source10 over conductor 12 to the center-tap of primary winding 20,.and over one half of the primary winding 20 to the collector of the ON transistor. Assuming for the moment that transistor 28 is on, current will flow through conductor 28A, from the collector to the emitter of transistor 28, and through resistor 31 and over conductor 14 to the negative terminal of DC source 10. If transistor 30 is on, current will flow through conductor 19, and from the collector to the emitter of transistor 30, through resistor 31, and over conductor 14 to the negative terminal of DC source 10. The result is that current flow is switched back and forth between transistors 28, and 30 at a frequency expressed by equation, 1.

During any one half cycle, the magnitude of the collector current of the on transistor is approximately The collector current or primary current is shown in FIG. 2A along with its relationship to the feedback voltage e,. -As shown, the primary current is made up of two components. One component is fixed by the setting of the control voltage E, provided by control voltage circuit 34, and the other of which component is proportional to the feedback voltage e, (which occurs across winding 21 which in turn is proportional to the voltage on the primary and secondary windings 20, 22 of transformer 26). The effect of transistors 28 and 30 conducting current alternately through the biphase primary winding 20 is the current i,,, as shown in FIG. 2A which flows through N, turns (N, is the number of turns between the center tap and one side of winding 20). The waveform for current i, in FIG. 2A is the summation of a square wave of amplitude E /R and a sinusoid of amplitude e,/R

At this point the equivalent circuit for the saturated current oscillator appears as shown in FIG. 2B. The primary current in FIG. 28 has been ratioed to the secondary winding 22 by the turns ratio ,,/N,. The magnetizing inductance, L and the equivalent core loss resistance, R have also been referred to the secondary winding 22. Magnetizing inductance L and capacitance C form a parallel resonant circuit and at the resonant frequency given by equation 1 presents an infinite impedance to the current source i,. 'The current flowing through resistance R which represents the core loss is proportional to voltage e in FIG. 23. By equating the voltage-proportional component of to the current required by R the core loss can be compensated for and the remaining load current i; will be independent of e or R It can be shown that the constant current characteristic will be realized if the following relation is satisfied:

Re a p f c Also, since the tuned transformer bypasses everything but the fundamental component of i,,, the load current is related to the control voltage by terminal on said source, controli mea ns'for providing a control voltage tothe center tap of said feedback winding, and output means including said output winding MODIFICATIONS As shown in FIG. 3, the basic circuit of FIG. 1 which supplies constantcurrent at high voltage to an AC load may also be used to provide a DC constant current. As there shown, a rectifier circuit 40 (or multiplier) may be connected to the output of-the saturated current oscillator 16 to provide a constant current DC output.

If a highly regulated DC output is desired, a sensing unit 42 (current or voltage) is connected as shown in FIG. 4 to sense the output of the rectifier 40, and the sensed signal is fed as one input to a feedback amplifier 44. A reference voltage E,- which may be adjustable is fed to a second input on the feedback amplifier 44, and the output of the feedback amplifier 44 is connected over conductor 13 to the center-tap of the feedback winding 21 in the saturated current oscillator 16in lieu of the signal from control circuit 34.

In an AC application, in which current regulations of a higher order are desired the circuit shown in FIG. 4 is modified by eliminating rectifier circuit 40. Other modifications and applications of the novel circuit of the invention will be apparentto the skilled, and such modifications and applications are considered to be within the scope of the invention.

What is claimed is:

1. In a constant current-supply circuit for providing constant current to aload having variable resistance characteristicscomprising input means for connecting said circuit to the first and second terminals of aDC power source, a first and second transistor, each of which has a collector, base and emitter element, transformer means having a center-tapped primary winding, a center-tapped feedback winding, an output winding, capacitor means connected across one of said transformer windings, means connecting the center tap of "said primary winding over said inputmeans to the first terminal of said source, means connecting the collector elements of said first and secondtransistors to said primary winding on opposite sides of said center tap, meansconnecting said base elements to-said feedback winding on opposite sides of said center'tap, common resistance means connecting the emitter elements of said first and second transistors common'tothe second for deriving an output for said load, said capacitor meansand the magnetizing inductance of said transformer means providing at least a part of a tank circuit for effecting operation of said transistors in an oscillatory mode, the circuit components including said common resistance means, the number of turns on said transformer output winding, the. number of turns on said center-tapped primary winding, and thenumber of turns on said feedback winding being of a valueto provide a control voltage to the base elements of saidtransistors which has an amplitude. whichvariesthe ,level of oscillation of the signal output from said transistors by a value to compensate for the changing core loss of the transformer which occurs with changes in the resistance load thereby maintaining the load current constant.

2. A constant current supply circuit as set forth in claim 1 in which said control means for providing a control voltage to the center tap of said feedback winding comprises a constant voltage reference source, and means for adjusting thebasic output control voltage to said center tap to a desired value to thereby adjust the value of current supplied to said load by said constantcurrent supply circuit.

3. A constant current oscillatorcircuit as set forth in claim 1 in which the components are selected'to provide the relationship.

R N, 2 N,,N,R

wherein R comprises said common resistance means, 7

N, comprises the number of turns on said transformer output winding, N comprises the number of turns on said center-tapped primary winding, R comprises the equivalent core loss resistance of said transformer, and N; comprises the number of turns on said feedback winding.

4. A constant current supply circuit as set forth in claim 1 in which said control means provides a control voltage to the center tap of said feedback rewinding which has a value wherein R, comprises said common resistance means, N, comprises the number of turns in said secondary winding, and N, comprises the number of turns in said primary winding and i, comprises the value of the load current.

5. A constant current supply circuit as set forth in claim 1 in which said output winding is a high voltage winding and said tank circuit conforms to the equation wherein L,, comprises the magnetizing inductance of said high voltage transformer; C is the value ofsaid-capacitor means and C comprises the distributed capacitance of the high voltage winding and wiring capacitance to ground.

6. A constant current supply circuit as set forth in claim 1 in which said control means includes sensor means for sensing a changing characteristic in said output for said load, and afeedback amplifier connected between said sensor means and said center tap on said feedback winding.

7. A constant current supply circuit as set forth in claim 6 in which said control means includes rectifier means connected between said output winding and said load, and said sensor means in said control means senses the DC output provided by said rectifier means to said load.

8. In a constant current supply circuit for providing constant current to a load having variable resistance characteristics comprising input means for connecting said circuit to the first and second terminals of a DC a power source, a first and second transistor each having base, emitter and collector elements, means for driving said transistors in an oscillatory mode including transformer means having a center-tapped primary winding the opposite ends of which are connected to the collecvoltage control circuit, control means for connecting the constant voltage output from said control circuit to the center tap of said feedback winding, means connecting a feedback voltage to the base elements of said transistors whichis derived across said feedback winding, and means connecting a common resistance from said emitter elements of said transistors to said second terminal of said source, the circuit components including said common resistance, the number of turns on said transformer output winding, the number of turns on said center-tapped primary winding, and the number of turns on said feedback winding being of a value to provide a control voltage to the base elements of the transistors which has an amplitude which varies the level of oscillation of the signal output from said transistors by a value to compensate for the changing core loss of the transofrrner which occurs with changes in the resistance load thereby maintaining the load current constant.

222 33 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 7'75 702 v Dated November 27 1973 Inventor(s) Kenneth Wallace It is certified that error appears in the above-identified petent I and that said Letters Patent are hereby corrected as shown below':

Column 6, line 43, "rewinding" should be winding line 46, in the formula, an sign should appear .II II after E V v v Signed and sealed this 9thday of J uly 197 (SEAL) Attest:

MCCOY M. GIBSON, JR. v I C. MARSHALL DANN Attesting Officer "Commissioner of Patents 

1. In a constant current supply circuit for providing constant current to a load having variable resistance characteristics comprising input means for connecting said circuit to the first and second terminals of a DC power source, a first and second transistor, each of which has a collector, base and emitter element, transformer means having a center-tapped primary winding, a center-tapped feedback winding, an output winding, capacitor means connected across one of said transformer windings, means connecting the center tap of said primary winding over said input means to the first terminal of said source, means connecting the collector elements of said first and second transistors to said primary winding on opposite sides of said center tap, means connecting said base elements to said feedback winding on opposite sides of said center tap, common resistance means connecting the emitter elements of said first and second transistors common to the second terminal on said source, control means for providing a control voltage to the center tap of said feedback winding, and output means including said output winding for deriving an output for said load, said capacitor means and the magnetizing inductance of said transformer means providing at least a part of a tank circuit for effecting operation of said transistors in an oscillatory mode, the circuit components including said common resistance means, the number of turns on said transformer output winding, the number of turns on said center-tapped primary winding, and the number of turns on said feedback winding being of a value to provide a control voltage to the base elements of said transistors which has an amplitude which varies the level of oscillation of the signal output from said transistors by a value to compensate for the changing core loss of the transformer which occurs with changes in the resistance load thereby maintaining the load current constant.
 2. A constant current supply circuit as set forth in claim 1 in which said control means for providing a control voltage to the center tap of said feedback winding comprises a constant voltage reference source, and means for adjusting the basic output control voltage to said center tap to a desired value to thereby adjust the value of current supplied to said load by said constant current supply circuit.
 3. A constant current oscillator circuit as set forth in claim 1 in which the components are selected to provide the relationship. ReNs 2 NpNfRc wherein Re comprises said common resistance means, Ns comprises the number of turns on said transformer output winding, Np comprises the number of turns on said center-tapped primary winding, Rc comprises the equivalent core loss resistance of said transformer, and Nf comprises the number of turns on said feedback winding.
 4. A constant current supply circuit as set forth in claim 1 in which said control means provides a control voltage to the center tap of said feedback rewinding which has a value Ec ((ReNs pi )/(2 Square Root 2Np ))i1 wherein Re comprises said common resistance means, Ns comprises the number of turns in said secondary winding, and Np comprises the number of turns in said primary winding and i1 comprises the value of the load current.
 5. A constant current supply circuit as set forth in claim 1 in which said output winding is a high voltage winding and said tank circuit conforms to the equation omega 2 ( ( 1 ) )/(Lm(Co + C1)) wherein Lm comprises the magnetizing inductance of said high voltage transformer; Co is the value of said capacitor means and C1 comprises the distributed capacitance of the high voltage winding and wiring capacitance to ground.
 6. A constant current supply circuit as set forth in claim 1 in which said control means includes sensor means for sensing a changing characteristic in said output for said load, and a feedback amplifier connected between said sensor means and said center tap on said feedback winding.
 7. A constant current supply circuit as set forth in claim 6 in which said control means includes rectifier means connected between said output winding and said load, and said sensor means in said control means senses the DC output provided by said rectifier means to said load.
 8. In a constant current supply circuit for providing constant current to a load having variable resistance characteristics comprising input means for connecting said circuit to the first and second terminals of a DC power source, a first and second transistor each having base, emitter and collector elements, means for driving said transistors in an oscillatory mode including transformer means having a center-tapped primary winding the opposite ends of which are connected to the collectors of said transistors, a center-tapped feedback winding, and an output winding, means for connecting said first terminal of said DC source to the center tap of said primary winding, means connecting the output of said transformer output winding to said load, a constant voltage control circuit, control means for connecting the constant voltage output from said control circuit to the center tap of said feedback winding, means connecting a feedback voltage to the base elements of said transistors which is derived across said feedback winding, and means connecting a common resistance from said emitter elements of said transistors to said second terminal of said source, the circuit components including said common resistance, the number of turns on said transformer output winding, the number of turns on said center-tapped primary winding, and the number of turns on said feedback winding being of a value to provide a control voltage to the base elements of the transistors which has an amplitude which varies the level of oscillation of the signal output from said transistors by a value to compensate for the changing core loss of the transofrmer which occurs with changes in the resistance load thereby maintaining the load current constant. 